The NYT has various brain trusters puzzle their puzzlers over this question:

Throughout the presidential campaign, pundits zeroed in on Latino voters: why President Obama might lose them, how Mitt Romney could woo them, whether they would even vote this year. In the end, the turnout set records, and Latinos overwhelmingly favored Obama. What never changed was the tendency to discuss Latino voters as a bloc. But the talking heads on TV didn’t devote air time to “the Irish vote” or “Korean American turnout.”

Why are some immigrants and their descendants considered simply “American,” while others are still thought of as “outsiders”? How does an immigrant group come to be thought of as native?

Hispanics are becoming an unmeltable ethnicity because Hispanics are legally classified by the Office of Management & Budget as entitled to affirmative action, while Irishmen are not. *

Of course, contrary to this question about why nobody talks about Korean American turnout, we do talk about "Asian turnout." A major reason is that Asians are entitled to some affirmative action benefits, but not to others. So they fall in between Latino immigrants and European immigrants in salience.

* Yes, I realize that raises all sorts of fascinating questions about the affirmative action eligibility of Charlie Sheen ** v. his brother Emilio Estevez. Fortunately, the Pew Hispanic Center can answer all your questions.

** Have you noticed how Charlie Sheen (above) increasingly looks like the Notre Dame Fighting Irish football team logo?